1. Field of the Invention
The present invention relates to an image display device and, more particularly, to a projection type image display device which is designed to enhance the quality of image by using the image division and shift method.
2. Discussion of Related Art
In general, flat panels, especially projection type image display devices with a liquid crystal display panel are implemented by real and virtual image techniques.
The real image technique is a method of displaying a real image by using an optical system as shown in FIG. 1. The optical system comprises a liquid crystal display panel 11, an eyepiece lens 12, and a screen 13. The liquid crystal panel 11 is positioned out of the focal distance f of the eyepiece lens 12 to display the real image projected on the screen 13.
The virtual image technique is a method of displaying a virtual image on a screen. FIG. 2 is a schematic of an optical instrument of the projection type image display device using a general virtual image technique. The optical instrument comprises an eyepiece 14, a liquid crystal display panel 15, and a screen 16. Unlike the real image technique, the liquid crystal panel 15 is positioned within the focal distance f of the eyepiece lens 14 to project the virtual image through the liquid crystal display panel 15 and display it on the screen 16.
The image of the liquid crystal display panel is projected on the screen in either case.
The type of the liquid crystal display panel is classified by the pattern of an array of picture elements; delta and stripe type patterns.
In the delta type pattern, as shown in FIG. 3, RGB color filters are horizontally arranged in sequence. The picture elements of one color in a horizontal line are horizontally moved by three halves the picture element in the adjacent horizontal lines.
Similarly, RGB color filters are horizontally arranged in sequence in the stripe type pattern, as shown in FIG. 4. But, the picture elements of one color in a horizontal line are located at the same vertical position in the adjacent horizontal lines.
Beside the liquid crystal display panel, flat panels to project an image include PDP, FED, and DMD panels. For all the flat panels, the image is projected in a larger size on the screen.
The flat panel comprises an opening section that allows a ray to pass therethrough, and opaque zones. The opaque zones are unavoidably formed from the circuits for energizing each picture element and other structural constructions constituting the panel.
For example, when an image by red, green and blue picture elements is enlarged and projected, a part of the image is not displayed on the screen due to the opaque zones between the picture elements.
FIG. 5 shows the intervals formed by the opaque zones 20 between the red, green and blue picture elements 17, 18 and 19 on the screen, where the opaque zones 20 on the screen are caused by those between the three picture elements on the liquid crystal display panel.
The picture elements are recognized as they are separated with the opaque zones unless they are observed at a good distance from the screen. If the picture elements that constitute an image are recognized in the case that they are not observed at a good distance from the screen, the image formed appears unnatural and rough. The intervals formed by the opaque zones are shown as a mosaic pattern in the image on the screen, which deteriorates the quality of image.
Since the light generated from the light source is screened by the opaque zones, there may be a loss of the quantity of light passing through the light path.
To eliminate the deterioration of the quality of image due to the opaque zones, as shown in FIG. 5, the picture elements 17, 18 and 19 are divided into two parts: original picture elements 17, 18 and 19 and duplication picture elements 17a, 18a and 19a and the duplication picture elements are shifted to such a distance that the opaque zones are eliminated with them. Both images by the original and shifted duplication picture elements are simultaneously displayed on the screen.
A plurality of images duplicated are shifted in different directions and to a distance shorter than the cycle P.sub.x of the picture element so as to eliminate the opaque zones. The cycle P.sub.x of the picture element is the distance between adjacent two picture elements and the horizontal pitch of the picture element on a liquid crystal display panel.
The key point of this is that the picture element (image) is duplicated into a plurality of picture elements (images) and the images by the original and duplication picture elements are projected onto the screen, thus eliminating the opaque zones to prevent the deterioration of the quality of image.
A wavefront division method is used to divide the wave of light propagating from the image on the LCD panel into a plurality of duplication images. FIG. 6 is a schematic of a projection type image display device using a flat plate in accordance with an example of the prior art, disclosed in U.S. Pat. No. 5,250,967. FIG. 7 is a schematic of a projection type image display device using a polyhedral prism in accordance with another example of the prior art, disclosed in U.S. Pat. No. 5,005,968. In the above mentioned examples, an optical unit is provided near the iris of the projection optical system, to divide the image into a plurality of images to a designated distance less than the cycle of the picture element.
Referring to FIG. 6, the projection type image display device using a flat plate comprises a light source 21, a liquid crystal display panel 22, an iris 23, a polyhedral flat plate 24, a projecting lens 25, and a screen 26. The liquid crystal display panel 22 produces an image to be projected onto the screen 26. The light containing image information is incident on the polyhedral flat panel 24 via the iris 23. The light on the polyhedral flat panel 24 is divided into a plurality of images, being projected onto the screen 26 through the projecting lens 25. Thus formed duplication image is shown in FIG. 5.
As shown in FIG. 7, the incident light is however scattered at border lines (edges) 24a, 24b and 24c of the polyhedral flat plate 24. Thus the quality of image is deteriorated irrespective of the elimination of the opaque zones by the projection type image display device in FIG. 6.
Since the polyhedral flat plate 24 must be inserted before the iris of the projection optical system, the installation of the projecting lens and image projection display system is limited by the configuration of the projection optical system. The polyhedral flat plate 24 to attain an accurate quantity of division and shift is also hard to manufacture.
Referring to FIG. 8, the conventional projection type image display device using a polyhedral prism comprises a light source 30, a liquid crystal display panel 31, an iris 32, a projecting lens 33, a polyhedral prism 34, and a screen 35. The liquid crystal display panel 31 produces an image to be projected onto the screen 35. The light containing the image information is incident upon the polyhedral prism 34 via the iris 32 and projecting lens 33. The light on the polyhedral prism 34 is divided into an original image by the original picture elements and a duplication image by the duplication picture elements, and projected onto the screen 26. Thus formed duplication image is shown in FIG. 5.
In a similar manner as illustrated in FIG. 7, the incident light is divided and shifted by a plurality of faces of the polyhedral prism 34 to generate an original image and a duplication image 27b. The projection type display device using a polyhedral prism as shown in FIG. 8 also has the same problems with the projection type display device using a polyhedral flat plate in FIG. 6.
Further another example of the prior art to eliminate the opaque zones is disclosed in JP Publication No. 8-43764, dividing the picture elements into the duplication picture elements and projecting the image by the original picture elements and the duplication image shifted onto a screen. The duplication image is shifted to a distance as large as the opaque zones in one picture element that is only enough not to overlap the adjacent two picture elements. As described in FIG. 9, the duplication image 36a is shifted to a distance of maximum 1/2P.sub.x, that is, a half the cycle of the picture element.
Opaque zones due to another picture element of a different color cannot be removed at all. Referring to FIG. 9, when only R picture element is used, or the quantity of light is concentrated only R picture element, G and B picture elements serve as opaque zones, with respect to the R picture element. Due to the opaque zones, the image displayed is unnatural and the opaque zones appear in the image as a mosaic pattern. This problem pertaining the opaque zones is to unavoidable in the above mentioned examples disclosed in U.S. Pat. No. 5,250,967 and U.S. Pat. No. 5,005,968.
To eliminate the opaque zones, a micro lens array sheet (U.S. Pat. No. 5,300,942) or a grating sheet is suggested for further another example. Each of micro lens must be arranged exactly corresponding to each picture element on a liquid crystal display panel. The deterioration of the quality of image is not avoidable because of a scattering of light at the border portions of the micro lens. For the grating sheet, unwanted scattering of light occurs at the edge of the grating plate and a moire appears in the image due to the interference between the grating plate and LCD panel. The image formed by the conventional liquid crystal display panel must be observed at a distance from the screen, which requires sufficient space for watching the screen.